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Checking the automatic transmission fluid level...


The automatic transmission fluid level should be checked every 3 months or 5,000 kilometres (3,000 miles), whichever comes first.  Where a fluid leak is suspected or the vehicle is being driven more than usual, it should be checked more frequently.  If the fluid level is found to be low at the automatic transmission dipstick, more fluid should be added through the transmission filler tube where the dipstick normally resides, since a low fluid level can lead to slipping and premature wear inside the transmission.


Important points for checking the automatic transmission fluid level…

It is important to observe the following points when checking the transmission fluid level:

Read the manufacturer’s instructions for checking the automatic transmission fluid level very carefully…

The fluid level should be checked strictly according to the instructions in the owner’s manual for the vehicle, which are usually quite complicated and often entail shifting from Park to Low while the vehicle is stationery and back up again, pausing for about five seconds in each gear.

Automatic transmission fluid dipsticks have hot and cold markings…

Note carefully that many transmission fluid dipsticks have two sets of minimum and maximum markings on them – one set for cold fluid and one set for hot fluid, depending on whether the transmission is cold or hot (as defined by the owner’s manual for the vehicle) when the fluid level is being checked.  It is critically important to use the correct set of markings.

Maintain absolute cleanliness when wiping the automatic transmission dipstick…

The automatic transmission is more sensitive to dirt, dust and other contaminants such as human hair than probably any other component on the vehicle.  Contaminants such as these can exacerbate wear inside the transmission and cause problems with the operation of the precision-engineered components of the transmission.  For this reason, it’s very important to use an immaculately clean, lint-free cloth when wiping the dipstick.  It’s well worth checking it for fibres or other contaminants such as specks of dirt after wiping it and remove any that you find before returning it to the transmission filler tube so that no contaminants enter the transmission.


Important points for adding fluid to the automatic transmission…

The following points should be observed when adding fluid to the transmission:

Don’t place the dipstick down on any unclean surface where it can become contaminated...

When you place the dipstick aside in order to add fluid through the filler tube, it’s very important that you don’t put the dipstick down somewhere where it will attract dirt, dust or any other contaminants that it will then transfer back into the automatic transmission when you return it to its place in the filler tube.  This leaves only two options:

  1. Putting the dipstick in a clean, plastic bag such as a garbage bag that has the same length as it.


  2. Hanging the dipstick up in some way from the underside of the front of the bonnet which is my preferred option.  It’s a good idea to make a hook out of coathanger wire and attach it to the underside of the front of the bonnet permanently so that whenever you go to check the automatic transmission fluid level, it’s there ready for you to hang the dipstick on.  Little things like this can really streamline the process of checking the fluid level and make it much easier.

Be sure that the funnel and tubing you use to add fluid to the transmission is clean and free from all contaminants…

It’s very important that the plastic funnel and 8mm internal-diameter clear, plastic tubing that you use to add fluid to the transmission via the filler tube are free from dirt, dust and any other contaminants.  If they’re not, they should be discarded and replaced with a new funnel and new tubing.  It’s a good idea to keep them in a clean, labelled, snap-lock bag when they’re not in use to keep them free of dirt, dust and any other contaminants.

Automatic transmission fluid expands when it’s hot…

When adding fluid, it’s important to be aware that automatic transmission fluid expands when it’s hot.  Therefore if you checked the fluid level when the transmission was hot (as defined by the owner’s manual) as opposed to cold (as defined by the owner’s manual) then the amount of fluid that you need to add (which is cold fluid out of the container) will need to take into account the expansion of that cold fluid that will take place when it heats up to the same temperature as the hot fluid in the hot transmission.  For this reason, when adding fluid to a hot transmission, it’s very important to only add a small amount of fluid at a time, then return the dipstick to the filler tube, run the vehicle according to the instructions in the owner’s manual and re-check the fluid level.  Failure to add only a small amount of fluid at a time will inevitably result in over-filling the automatic transmission which can cause transmission damage if you don’t extract the excess fluid.

Innovative method for extracting excess automatic transmission fluid…

If you end up adding too much fluid to the transmission so that it becomes over-filled, you can extract the excess fluid using a 3mm internal-diameter plastic tube on the end of a plastic syringe through the filler tube or by loosening the drain-plug underneath the transmission, which is the messier option.  If you use the method of the tubing and syringe, it’s important to be aware of the importance of preventing the tubing from falling into the transmission through the filler tube if it accidentally becomes detached from the syringe so that it is difficult to recover.  A good way to prevent this from happening is to make the tubing so long that it can’t possibly fall into the filler tube.

Aim for a fluid level significantly above the minimum mark on the dipstick...

It’s best to aim for a fluid level which is some margin below the maximum mark on the dipstick simply because the maximum fluid level is so difficult to pick.  However, the fluid level must absolutely be above the minimum mark on the dipstick.  A good compromise is to aim for a fluid level three-quarters of the way between the low and high marks on the dipstick, but closer to the high mark than the low mark.


Checking the automatic transmission fluid level is a very important task to get right...

Checking the transmission fluid level and, particularly, topping it up to get the correct fluid level is probably one of the most difficult parts of routine auto maintenance.  I personally find it to be very challenging.  It’s a time-consuming process which requires patience because you need to go through the whole process of checking at the dipstick, adding the fluid, replacing the dipstick and then shifting through the gears more than once to get the fluid level right.  There’s no easy way around it and there are no short-cuts.  It’s critically important to take the time to get the fluid level right because the automatic transmission is a very expensive component to repair and both an under-filled and an over-filled transmission can result in expensive problems if left uncorrected.



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Copyright 2016 Andrew Mackinnon.  All rights reserved.